Submitted by the present inventor and described in JP-A-2001-161094 is a control method, for an alternating-current motor, whereby are provided a power converter, for outputting power to the alternating-current motor, and a current controller, for controlling the current output by the power converter based on a signal indicating a deviation between a current instruction signal and a detection signal indicating the current output by the power converter, and whereby a velocity detector and a voltage detector are not provided. According to this control method, provided are the power converter, for outputting power to the alternating-current motor, and the power controller, for controlling the current output by the power converter based on a signal indicating a deviation between a current instruction signal and a detection signal indicating the current output by the power converter, and when the alternating-current motor is in the free running state, an arbitrary direct current is supplied for a designated period of time, a frequency component that appears in the detection signal for the power output by the power converter is detected, and the velocity of the alternating-current motor is estimated based on the frequency component.
Also disclosed is a control method whereby, when an alternating-current motor is in the free running state, a current instruction signal described above is forcibly set to zero so as to adjust to zero the current for the alternating-current motor; and whereby the level of the remaining voltage, the phase and the angular velocity of the alternating-current motor are obtained based on an output voltage instruction signal that is obtained by using a calculation employing the current output by the current controller, and then, the rotational direction and the velocity of the alternating-current motor in the free running state are estimated, so that the alternating-current motor can be smoothly started in the free running state.
Furthermore, disclosed is a control method whereby, when an output voltage instruction signal, which is obtained by using a calculation based on the output of the current controller when the current control is performed with a current instruction signal of zero, is lower than an arbitrarily designated voltage level, the current control is halted and an instruction for a direct current having an arbitrary level is transmitted in an arbitrary direction for a designated period of time, and thereafter, an instruction for a current having an arbitrary level is transmitted in a direction for which the phase differs by 180° from the direction in which the direct current instruction is transmitted; whereby the current control is again performed for a designated period of time, and the frequency component that appears in the detected current value and the phase relationship are detected; and whereby the frequency component is estimated to be the velocity of the alternating-current motor and the rotational direction is estimated based on the phase relationship.
However, according to the method described in JP-2001-161094, when a residual high voltage remains in the alternating-current motor, a velocity differing greatly from the actual velocity of the alternating-current motor would be estimated to be due to the averse affect of the residual voltage. In this case, when the alternating-current motor is started while a frequency corresponding to the estimated velocity is designated for the power converter, a large current flows that produces a velocity near the velocity erroneously detected for the alternating-current motor, and the alternating-current motor can not smoothly be restarted.
When a response from the current controller is not satisfactory, it is difficult for the current of the alternating-current motor to be set to zero, the power converter falls into an overcurrent state, and the alternating-current motor can not smoothly be started.
Further, when the alternating-current motor is an induction motor, it is easy for the current of the induction motor to be reduced to zero because the residual voltage in the free running state is gradually reduced. But when the alternating-current motor is a permanent magnet synchronous motor, a high inductive voltage is generated in the free running state at a high velocity, and it is not easy for the current of the permanent magnet synchronous motor to be set to zero.
Furthermore, when the alternating-current motor is in the free running state at a high velocity, the detection resolution for a frequency that appears in a detected current value, or the amplitude of the signal of a frequency component that appears in a detected current value is reduced, so that the frequency can not be detected.
In addition, according to the control method for an alternating-current motor described in JP-A-2001-161094, when the alternating-current motor is in the free running state, an arbitrary direct current is supplied for a designated period of time. However, no specific explanation is given for the method for determining the designated period of time.
According to a control method for an alternating-current motor described in Japanese Patent Application No. 2002-80891, a predesignated frequency and a rotational direction that is detected are set for a frequency adjustment circuit, and when an input torque current detection value is positive, the output frequency is lowered, or when the torque current detection value is negative, the output frequency is increased. When the output frequency is adjusted in this manner, so that it nears the torque current detection value of 0, the output frequency of the alternating-current motor in the free running state can match the output frequency of the power converter, so that a smooth start is obtained.
However, in this case also, the alternating-current motor is not always smoothly restarted, even though the output frequency is adjusted so that it nears the torque current detection value of 0.
Therefore, in order to resolve these shortcomings, it is a first objective of the present invention to provide a sensorless vector control method for an alternating-current motor, whereby when a wrong rotational direction or a wrong velocity is estimated when restarting the alternating-current motor in the free running state, this can be determined to be an erroneous estimate and the alternating-current motor in the free running state can be smoothly restarted, and whereby when an alternating-current motor in the free running state is to be restarted, a period for the application of a direct current to the alternating-current motor is correctly designated, so that the alternating-current motor in the free running state can be smoothly restarted; and a control apparatus therefor.
It is a second objective of the present invention to provide a sensorless vector control method for an alternating-current motor, whereby a running operation can still be smoothly and appropriately continued when a response by a current controller is poor, or when the alternating-current motor is an induction alternating-current motor, or even a permanent magnet synchronous alternating-current motor; and a control apparatus therefor.
It is a third objective of the present invention to provide a sensorless vector control method for an alternating-current motor, whereby the following three points can be satisfied: when, after a current instruction signal is set to zero in order to adjust the current for the alternating-current motor to zero, current control is implemented to increase the response of a current controller and to avoid the entry into an overcurrent state of a power converter, so that the run-time operation can be smoothly continued; when, after an estimated velocity and rotational direction for the alternating-current motor are employed to provide a direct-current instruction for the alternating-current motor, the accuracy of the detection of a frequency is increased for the alternating-current motor in the free running state at a high velocity; and when the run-time operation continues smoothly even while the alternating-current motor is in the free running state at a high velocity; and to provide a control apparatus therefor.